In a coolant-hole equipped drill, a coolant hole is in general formed so as to have a circular cross section. In order to supply coolant in an increased amount and in an effective manner, for example, Patent Document 1 has proposed a coolant-hole equipped drill in which an axial cross section of the coolant hole is formed in such a droplet shape that the distance between the inner wall surfaces is gradually decreased from the approximate center of the coolant hole to the center of rotation. Patent Document 2 discloses a drill in which the axial cross section of the coolant hole is formed in an oval shape. Patent Document 3 discloses a drill in which at least an opening portion of the coolant hole is formed substantially in a triangular shape.
In the above-described coolant-hole equipped drills, for example, Patent Documents 4 to 6 have proposed ones in which three margin portions consisting of a first margin portion to a third margin portion are formed on an outer circumferential surface of a land portion between a pair of chip discharging grooves on an outer circumferential surface of a cutting edge portion. In the triple margin-type drills, the three margin portions guide the cutting edge portion stably to the hole-to-be-bored, thus making it possible to conduct highly accurate boring work.